1. Field of the Invention
The present invention relates to a piezoelectric ceramic including oxide consisted of a first element including lead (Pb), a second element including zinc (Zn), niobium (Nb), titanium (Ti) and zirconium (Zr), and oxygen (O) and to a piezoelectric device using the same.
2. Description of the Related Art
Presently, piezoelectric devices utilizing piezoelectric ceramics have been widely used in a wide variety of technical fields. The piezoelectric devices include ultrasonic oscillators, filters, piezoelectric transformers and so on. Of these piezoelectric devices, the piezoelectric transformers recently attract attention. The piezoelectric transformers can be compact, thin and light compared to the conventional electromagnetic transformers. In addition, the piezoelectric transformers have characteristics of high-energy conversion efficiency and less noise.
A typical piezoelectric transformer using a rectangular piezoelectric ceramic plate comprises a primary (input) portion polarized in a direction of thickness and a secondary (output) portion polarized in the lengthwise direction. In the piezoelectric transformer, application of alternating voltage with one wavelength or a half wavelength resonance converts electrical energy to vibrational energy in the low impedance primary portion. The vibrational energy is propagated to the high impedance secondary portion and converted to electrical energy thereby high voltage is generated. In such a piezoelectric transformer, certain intensity, which avoids fracture by impulse and by excitation with large amplitude at high input voltage in a concentration point of stress called a node point, is required. In other words, a high mechanical strength is required for piezoelectric ceramics using for piezoelectric transformers in particular.
The mechanical strength of the piezoelectric transformers closely correlates with defects such as hole. In order to improve the mechanical strength, various studies have been made. For example, grain size diameter of ceramic is made at most 1 xcexcm by calcining raw material powder having a small particle diameter with at least 10 m2/g specific surface area at most a temperature of 650xc2x0 C. (disclosed in Publication of Unexamined Japanese Patent Application No. Hei 6-112542). The mechanical strength of the piezoelectric transformers can be improved by using a hot press as well.
Principal components of the piezoelectric ceramics are lead titanate, lead titanate zirconate or the like. Also, a second component, a third component or various additives are added to improve the properties of the piezoelectric ceramics. For example, manganese oxide and cobalt oxide are added to zinc niobic lead titanate zirconate (disclosed in Publication of Examined Japanese Patent Application No. Sho 54-18400), or strontium, barium or the like substitute for lead in part (disclosed in Publication of Unexamined Japanese Patent Application No. Sho 62-154682). In addition, niobium oxide, antimony oxide and tantalum oxide are added to the composition mentioned in the Publication of Examined Japanese Patent Application No. Sho 54-18400 to improve the mechanical strength of the piezoelectric transformers (Publication of Japanese Patent No. 2957537).
However, it is difficult to handle the calcined powder in fabricating the piezoelectric ceramics by a method of improving the mechanical strength using the ground-calcined powder. In addition, a step of grinding calcined powder is required. This deteriorates the manufacturing efficiency. In a method using a hot press, it takes longer time to manufacture compared to the usual manufacturing process with no use of hot press and require high cost of equipment. This results in high product price. As a simple and low-cost solution, adjustment of the composition of piezoelectric ceramics is desired to improve the mechanical strength.
The present invention is designed to overcome the foregoing problems. It is an object of the invention to provide a piezoelectric ceramic capable of improving the mechanical strength simply and at a low cost and a piezoelectric device using the same.
A piezoelectric ceramic of the invention includes oxide consisted of a first element, a second element and oxygen. The first element includes at least lead in the group comprising lead, calcium (Ca), strontium (Sr) and barium (Ba), the composition is represented by the chemical formula 1:
xe2x80x83Pb1xe2x88x92aAaxe2x80x83xe2x80x831
wherein A represents at least one element in the group comprising Ca, Sr and Ba, and a is within a range of 0xe2x89xa6axe2x89xa60.1. The second element includes zinc, manganese (Mn), titanium and zirconium, and at least niobium in the group comprising niobium, antimony (Sb) and tantalum (Ta), the composition of these elements is represented by the chemical formula 2:
(Zn1/3Nb2/3)w(Mn1/3B2/3)xTiyZrzxe2x80x83xe2x80x832
wherein B represents at least one element in the group comprising Nb, Sb and Ta, and w, x, y and z are within a range of 0.2xe2x89xa6w/xxe2x89xa63.0, 0.1xe2x89xa6yxe2x89xa60.5 and 0.2xe2x89xa6zxe2x89xa60.6, respectively and w+x+y+z=1. A composition ratio of the first element to the second element (the first element/the second element) is at least 0.9 and less than 1.0 in the molar ratio.
The piezoelectric ceramic includes oxide having above-mentioned composition. This enables high mechanical strength and excellent piezoelectric properties.
The piezoelectric device of the invention utilizes the piezoelectric ceramic of the invention.
Other and further objects, features and advantages of the invention will appear more fully from the following description.